1. Field of the Invention
This invention relates to a full contact image sensor device which senses an image of objects without reduction of the image. The full contact image sensor device is utilized for small-sized facsimile terminal equipment, a bar code reader, and the like.
2. Description of the Related Art
Since a contact image sensor device can sense an image of objects without use of an optical system for reduction of the image, the length of an optical path can be shortened so that equipment comprising the contact image sensor device can be miniaturized. For this reason, recently, contact image sensor devices are widely used as an image sensing unit in small sized facsimile terminal equipment, a bar code reader, and the like.
Among various types of conventional contact type image sensor devices, one type of image sensor device uses an optical system without reduction or magnification such as, typically, a SELFOC lens to form an image on the surface of the sensor device. In this type of conventional contact type image sensor device, a clear image is obtained by providing a SELFOC lens on a light receiving surface of sensor elements and by focusing the image of the objects through the SELFOC lens on the sensor elements.
However, the device has defects in that thin type sensors cannot be manufactured due to use of the lens even though a lens having a short focal length is used, and if a lens having too short a focal length is used, the image gets dim due to color aberration. Especially, in a color sensor using the SELFOC lens, a lens having a long focal length must be used, so that the sensor cannot be miniaturized.
For this reason, a type of contact image sensor wherein the SELFOC lens is not used and the sensor elements macroscopically touch the objects to read the image has been developed aiming at lowering the cost of the sensor and miniaturizing the sensor device. This type of contact image sensor device is referred to hereinafter as a full contact image sensor device.
In the full contact image sensor device, a plurality of sensor elements are formed on a lower surface of a transparent plate. A document whose image is sensed is placed under the transparent plate and the sensor elements so that lower surfaces (light receiving surfaces) of the sensor elements come close to the document, and the document is illuminated from above an upper surface of the transparent plate. Light from a light source permeates through the transparent plate, passes through a space between the sensor elements, is irregularly reflected by the document, and reaches the receiving surface of the sensor elements.
If the distance between the document and the sensor elements is too short, the reflected light from the document reaches only the peripheral area of the receiving surface of the sensor elements.
Therefore, the conventional full contact type image sensor device is designed so that the distance between the sensor elements and the distance between the document and the sensor elements are long enough to ensure a sufficient amount of light. To this end, though the sensitivity of the sensor is ensured, reflected light from the document comes not only to the corresponding sensor elements but also to the neighboring sensor elements. This crosstalk degrades the MTF (Modulation Transfer Function) of the image sensor device.
U.S. Pat. No. 4,149,197 and Japanese Unexamined Patent Publication (Kokai) No. 59-48954 disclose a sensor element having a central aperture in order to ensure the sensitivity without degrading the MTF.
In the sensor device disclosed in the aforementioned references, an aperture must be formed in the center of each sensor element. When the aperture bounded by a closed curve is formed in a wet etching process which is usually used, a "puddle" of liquid such as etchant used in the wet etching is created on a pattern bounded by the closed curve, so that it is not easy to form a uniform and clear aperture with good